Optical film including collimating reflective polarizer

ABSTRACT

Optical films are disclosed. More particularly, optical films including a collimating reflective polarizer are disclosed. The optical films are useful in backlights, and in particular backlight recycling cavities. Constructions suitable with both edge-lit and direct-lit backlights are disclosed.

BACKGROUND

Reflective polarizers substantially transmit light having onepolarization state while substantially reflecting light having anorthogonal polarization state and are used in backlights for displaydevices. In some cases, reflective polarizers are coupled with otherreflective surfaces to create a light recycling cavity. Microreplicatedfilms having a plurality of light directing elements may be used tochange the angular distribution of light.

SUMMARY

In one aspect, the present disclosure relates to an optical film. Theoptical film includes a collimating reflective polarizer and an array ofconcave microlenses disposed on a major surface of the collimatingreflective polarizer. In some embodiments, the collimating reflectivepolarizer has a transmission along a pass axis at normal incident ofTpassnormal for p-pol light and a transmission along a pass axis at 60degrees incidence of Tpass60 for p-pol light, and a ratio of Tpass60 toTpassnormal is less than 0.75. In some embodiments, the ratio of Tpass60to Tpassnormal is less than 0.60 or 0.50. In some embodiments, the arrayof concave microlenses is randomly arranged. In some embodiments, thearray of concave microlenses is regularly arranged. In some embodiments,each microlens in the array of concave microlenses has an aspect ratioof about 0.5. In some embodiments, each microlens in the array ofconcave microlenses has an aspect ratio of about 0.3. In someembodiments, each microlens in the array of concave microlenses has anaspect ratio of less than about 0.5. In some embodiments, the opticalfilm further includes microfeatures disposed on a surface of thecollimating reflective polarizer not disposed on the array of concavemicrolenses. The microfeatures may include beads. In some embodiments, amean diameter of the beads is about 8 μm. In some embodiments, themicrofeatures include convex lenses or prisms. In some embodiments, theoptical film has a thickness of less than 420 μm. In some embodiments,an edge-lit backlight assembly includes the optical film and alightguide, where the lightguide is disposed proximate the array ofconcave microlenses. In some embodiments, a direct-lit backlightassembly includes the optical film and one or more light sources, wherethe one or more light sources are disposed proximate the array ofconcave microlenses.

In another aspect, the present disclosure relates to an optical filmincluding an optical substrate having a first and second major surface,a collimating reflective polarizer disposed on the first major surfaceof the optical substrate, and an array of concave microlenses disposedon the second major surface of the optical substrate. In someembodiments, the optical film further includes an adhesive between theoptical substrate and the collimating reflective polarizer. In someembodiments, the collimating reflective polarizer has a transmissionalong a pass axis at normal incident of Tpassnormal for p-pol light anda transmission along a pass axis at 60 degrees incidence of Tpass60 forp-pol light, and a ratio of Tpass60 to Tpassnormal is less than 0.75. Insome embodiments, the ratio of Tpass60 to Tpassnormal is less than 0.60or 0.50. In some embodiments, the array of concave microlenses israndomly arranged. In some embodiments, the array of concave microlensesis regularly arranged. In some embodiments, each microlens in the arrayof concave microlenses has an aspect ratio of about 0.5. In someembodiments, each microlens in the array of concave microlenses has anaspect ratio of about 0.3. In some embodiments, each microlens in thearray of concave microlenses has an aspect ratio of less than about 0.5.In some embodiments, the optical film further includes microfeaturesdisposed on a surface of the collimating reflective polarizer notdisposed on the optical substrate. The microfeatures may include beads.In some embodiments, a mean diameter of the beads is about 8 μm. In someembodiments, the microfeatures include convex lenses or prisms. In someembodiments, the optical film has a thickness of less than 420 μm. Insome embodiments, an edge-lit backlight assembly includes the opticalfilm and a lightguide, where the lightguide is disposed proximate thearray of concave microlenses. In some embodiments, a direct-litbacklight assembly includes the optical film and one or more lightsources, where the one or more light sources are disposed proximate thearray of concave microlenses.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top perspective schematic view of a multilayer optical film.

FIG. 2 is a side elevation cross-section of an optical film including acollimating reflective polarizer.

FIG. 3 is a side elevation cross-section of another optical filmincluding a collimating reflective polarizer.

FIG. 4 is a side elevation cross-section of another optical filmincluding a collimating reflective polarizer.

FIG. 5 is a side elevation cross-section of a portion of an edge-litbacklight including the optical film of FIG. 2 .

FIG. 6 is a side elevation cross-section of a portion of a direct-litbacklight including the optical film of FIG. 2 .

FIG. 7 is a graph showing the layer thickness profile of the collimatingreflecting polarizer in Comparative Example C-2.

DETAILED DESCRIPTION

FIG. 1 is a top perspective schematic view of a multilayer optical film.Multilayer optical film 10 includes first layers 12 and second layers14.

Multilayer optical films, i.e., films that provide desirabletransmission and/or reflection properties at least partially by anarrangement of microlayers of differing refractive index, are known. Ithas been known to make such multilayer optical films by depositing asequence of inorganic materials in optically thin layers (“microlayers”)on a substrate in a vacuum chamber. Inorganic multilayer optical filmsare described, for example, in textbooks by H. A. Macleod, Thin-FilmOptical Filters, 2nd Ed., Macmillan Publishing Co. (1986) and by A.Thelan, Design of Optical Interference Filters, McGraw-Hill, Inc.(1989).

Multilayer optical films have also been demonstrated by coextrusion ofalternating polymer layers. See, e.g., U.S. Pat. No. 3,610,729 (Rogers),U.S. Pat. No. 4,446,305 (Rogers et al.), U.S. Pat. No. 4,540,623 (Im etal.), U.S. Pat. No. 5,448,404 (Schrenk et al.), and U.S. Pat. No.5,882,774 (Jonza et al.). In these polymeric multilayer optical films,polymer materials are used predominantly or exclusively in the makeup ofthe individual layers. Such films are compatible with high volumemanufacturing processes and can be made in large sheets and roll goods.

A multilayer optical film includes individual microlayers havingdifferent refractive index characteristics so that some light isreflected at interfaces between adjacent microlayers. The microlayersare sufficiently thin so that light reflected at a plurality of theinterfaces undergoes constructive or destructive interference in orderto give the multilayer optical film the desired reflective ortransmissive properties. For multilayer optical films designed toreflect light at ultraviolet, visible, or near-infrared wavelengths,each microlayer generally has an optical thickness (a physical thicknessmultiplied by refractive index) of less than about 1 μm. Thicker layersmay be included, such as skin layers at the outer surfaces of themultilayer optical film, or protective boundary layers (PBLs) disposedwithin the multilayer optical films, that separate coherent groupings(referred to herein as “packets”) of microlayers.

For polarizing applications, e.g., for reflective polarizers, at leastsome of the optical layers are formed using birefringent polymers, inwhich the polymer's index of refraction has differing values alongorthogonal Cartesian axes of the polymer. Generally, birefringentpolymer microlayers have their orthogonal Cartesian axes defined by thenormal to the layer plane (z-axis), with the x-axis and y-axis lyingwithin the layer plane. Birefringent polymers can also be used innon-polarizing applications.

Referring now to FIG. 1 , a schematic perspective view of an exemplaryoptical repeat unit (ORU) of a multilayer optical film is illustrated.FIG. 1 depicts only two layers of a multilayer optical film 10, whichcan include tens or hundreds of such layers arranged in one or morecontiguous packets or stacks. The film 10 includes individualmicrolayers 12, 14, where “microlayers” refer to layers that aresufficiently thin so that light reflected at a plurality of interfacesbetween such layers undergoes constructive or destructive interferenceto give the multilayer optical film the desired reflective ortransmissive properties. The microlayers 12, 14 can together representone optical repeat unit (ORU) of the multilayer stack, an ORU being thesmallest set of layers that recur in a repeating pattern throughout thethickness of the stack. The microlayers have different refractive indexcharacteristics so that some light is reflected at interfaces betweenadjacent microlayers. For optical films designed to reflect light atultraviolet, visible, or near-infrared wavelengths, each microlayertypically has an optical thickness (i.e., a physical thicknessmultiplied by refractive index) of less than about 1 micrometer.

In some cases, the microlayers 12, 14 have thicknesses and refractiveindex values corresponding to a ¼-wave stack, i.e., arranged in opticalrepeat units or unit cells each having two adjacent microlayers of equaloptical thickness (f-ratio=50%), such optical repeat unit beingeffective to reflect by constructive interference light whose wavelengthλ is twice the overall optical thickness of the optical repeat unit.Other layer arrangements, such as multilayer optical films having2-microlayer optical repeat units whose f-ratio is different from 50%,or films whose optical repeat units include more than two microlayers,are also known. These optical repeat unit designs can be configured toreduce or to increase certain higher-order reflections. See, e.g., U.S.Pat. No. 5,360,659 (Arends et al.) and U.S. Pat. No. 5,103,337 (Schrenket al.). Thickness gradients along a thickness axis of the film (e.g.,the z-axis) can be used to provide a widened reflection band, such as areflection band that extends over the entire human visible region andinto the near infrared so that as the band shifts to shorter wavelengthsat oblique incidence angles the microlayer stack continues to reflectover the entire visible spectrum. Thickness gradients tailored tosharpen band edges, i.e., the wavelength transition between highreflection and high transmission, are discussed in U.S. Pat. No.6,157,490 (Wheatley et al.).

Further details of multilayer optical films and related designs andconstructions are discussed in U.S. Pat. No. 5,882,774 (Jonza et al.)and U.S. Pat. No. 6,531,230 (Weber et al.), PCT Publications WO 95/17303(Ouderkirk et al.) and WO 99/39224 (Ouderkirk et al.), and thepublication entitled “Giant Birefringent Optics in Multilayer PolymerMirrors”, Science, Vol. 287, March 2000 (Weber et al.). The multilayeroptical films and related articles can include additional layers andcoatings selected for their optical, mechanical, and/or chemicalproperties. For example, a UV absorbing layer can be added at theincident side of the film to protect components from degradation causedby UV light. The multilayer optical films can be attached tomechanically reinforcing layers using a UV-curable acrylate adhesive orother suitable material. Such reinforcing layers may comprise polymerssuch as PET or polycarbonate, and may also include structured surfacesthat provide optical function such as light diffusion or collimation,e.g. by the use of beads or prisms. Additional layers and coatings canalso include scratch resistant layers, tear resistant layers, andstiffening agents. See, e.g., U.S. Pat. No. 6,368,699 (Gilbert et al.).Methods and devices for making multilayer optical films are discussed inU.S. Pat. No. 6,783,349 (Neavin et al.).

The reflective and transmissive properties of multilayer optical filmare a function of the refractive indices of the respective microlayersand the thicknesses and thickness distribution of the microlayers. Eachmicrolayer can be characterized at least in localized positions in thefilm by in-plane refractive indices n_(x), n_(y), and a refractive indexn_(z) associated with a thickness axis of the film. These indicesrepresent the refractive index of the subject material for lightpolarized along mutually orthogonal x-, y-, and z-axes, respectively.For ease of explanation in the present patent application, unlessotherwise specified, the x-, y-, and z-axes are assumed to be localCartesian coordinates applicable to any point of interest on amultilayer optical film, in which the microlayers extend parallel to thex-y plane, and wherein the x-axis is oriented within the plane of thefilm to maximize the magnitude of Δn_(x). Hence, the magnitude of Δn_(y)can be equal to or less than—but not greater than—the magnitude ofΔn_(x). Furthermore, the selection of which material layer to begin within calculating the differences Δn_(x), Δn_(y), Δn_(z) is dictated byrequiring that Δn_(x) be non-negative. In other words, the refractiveindex differences between two layers forming an interface areΔn_(j)=n_(1j)−n_(2j), where j=x, y, or z and where the layerdesignations 1,2 are chosen so that n_(1x)≥n_(2x), i.e., Δn_(x)≥0.

In practice, the refractive indices are controlled by judiciousmaterials selection and processing conditions. A multilayer film is madeby co-extrusion of a large number, e.g. tens or hundreds of layers oftwo alternating polymers A, B, typically followed by passing themultilayer extrudate through one or more multiplication die, and thenstretching or otherwise orienting the extrudate to form a final film.The resulting film is typically composed of many hundreds of individualmicrolayers whose thicknesses and refractive indices are tailored toprovide one or more reflection bands in desired region(s) of thespectrum, such as in the visible or near infrared. To achieve highreflectivities with a reasonable number of layers, adjacent microlayerstypically exhibit a difference in refractive index (Δn_(x)) for lightpolarized along the x-axis of at least 0.05. In some embodiments,materials are selected such that the difference in refractive index forlight polarized along the x-axis is as high as possible afterorientation. If the high reflectivity is desired for two orthogonalpolarizations, then the adjacent microlayers also can be made to exhibita difference in refractive index (Δn_(y)) for light polarized along they-axis of at least 0.05.

The '774 (Jonza et al.) patent referenced above describes, among otherthings, how the refractive index difference (Δn_(z)) between adjacentmicrolayers for light polarized along the z-axis can be tailored toachieve desirable reflectivity properties for the p-polarizationcomponent of obliquely incident light. To maintain high reflectivity ofp-polarized light at oblique angles of incidence, the z-index mismatchΔn_(z) between microlayers can be controlled to be substantially lessthan the maximum in-plane refractive index difference Δn_(x), such thatΔn_(z) 0.5*Δn_(x), or Δn_(z)≤0.25*Δn_(x). A zero or near zero magnitudez-index mismatch yields interfaces between microlayers whosereflectivity for p-polarized light is constant or near constant as afunction of incidence angle. Furthermore, the z-index mismatch Δn_(z)can be controlled to have the opposite polarity compared to the in-planeindex difference Δn_(x), i.e. Δn_(z)<0. This condition yields interfaceswhose reflectivity for p-polarized light increases with increasingangles of incidence, as is the case for s-polarized light.

The '774 (Jonza et al.) patent also discusses certain designconsiderations relating to multilayer optical films configured aspolarizers, referred to as multilayer reflecting or reflectivepolarizers. In many applications, the ideal reflecting polarizer hashigh reflectance along one axis (the “extinction” or “block” axis) andzero reflectance along the other axis (the “transmission” or “pass”axis). For the purposes of this application, light whose polarizationstate is substantially aligned with the pass axis or transmission axisis referred to as pass light and light whose polarization state issubstantially aligned with the block axis or extinction axis is referredto as block light. Unless otherwise indicated, pass light at 60°incidence is measured in p-polarized pass light. If some reflectivityoccurs along the transmission axis, the efficiency of the polarizer atoff-normal angles may be reduced, and if the reflectivity is differentfor various wavelengths, color may be introduced into the transmittedlight. Furthermore, exact matching of the two y indices and the two zindices may not be possible in some multilayer systems, and if thez-axis indices are not matched, introduction of a slight mismatch may bedesired for in-plane indices n1y and n2y. In particular, by arrangingthe y-index mismatch to have the same sign as the z-index mismatch, aBrewster effect is produced at the interfaces of the microlayers, tominimize off-axis reflectivity, and therefore off-axis color, along thetransmission axis of the multilayer reflecting polarizer.

Another design consideration discussed in '774 (Jonza et al.) relates tosurface reflections at the air interfaces of the multilayer reflectingpolarizer. Unless the polarizer is laminated on both sides to anexisting glass component or to another existing film with clear opticaladhesive, such surface reflections will reduce the transmission of lightof the desired polarization in the optical system. Thus, in some casesit may be useful to add an antireflection (AR) coating to the reflectingpolarizer.

Reflective polarizers are often used in visual display systems such asliquid crystal displays. These systems—now found in a wide variety ofelectronic devices such as mobile phones, computers including tablets,notebooks, and subnotebooks, and some flat panel TVs—use a liquidcrystal (LC) panel illuminated from behind with an extended areabacklight. The reflective polarizer is placed over or otherwiseincorporated into the backlight to transmit light of a polarizationstate useable by the LC panel from the backlight to the LC panel. Lightof an orthogonal polarization state, which is not useable by the LCpanel, is reflected back into the backlight, where it can eventually bereflected back towards the LC panel and at least partially converted tothe useable polarization state, thus “recycling” light that wouldnormally be lost, and increasing the resulting brightness and overallefficiency of the display.

FIG. 2 is a side elevation cross-section of an optical film including acollimating reflective polarizer. Optical film 200 includes collimatingreflective polarizer 210, microlens array 220 including lenslet 222,optical substrate 230, and adhesive 240.

Collimating reflective polarizer 210 may be any suitable thickness. Insome embodiments collimating reflective polarizer 210 may be amultilayer collimating reflective polarizer. For multilayer collimatingreflective polarizer, the layer thicknesses and indices of refractionmay be selected to provide better transmission of on-axis incident lightversus obliquely incident light. Combined with a back reflector,collimating reflective polarizers may recycle obliquely incident light.On subsequent passes, recycled light may be more likely to betransmitted on-axis, resulting in a collimating effect. As described,for example, in PCT Publication WO 2013/059225 A1 (Weber et al.), auseful indication of the potential degree of collimation of light from arecycling backlight using such films may be obtained from the ratio oftransmitted p-pol light at normal incidence (Tpassnormal) and 60 degreesincidence (Tpass60) of a given film. Useful collimating reflectivepolarizers may have ratios of Tpass60 to Tpassnormal of less than 0.75,0.6, 0.5 or lower.

Microlens array 220 includes a series of lenslet 222. Microlens array220—despite its label-need not include lenslets, but may instead oradditionally include any other microfeature, such as prisms, or spheres.In some embodiments, the microfeatures of microlens array 220 areinverted; that is, the feature's inverse shape is a lenslet (as shown inFIG. 2 ), sphere, or prism. In some embodiments the microfeatures mayextend in one in-plane direction; in other words, microlens array 220may be a one dimensional array. In some embodiments, microlens array 220may be a two dimensional array. Microlens array 220 may be arrangedregularly, randomly, or pseudorandomly and may have any suitable pitch.In some embodiments, lenslet 222 are closely packed; in some embodimentslenslet 222 are spaced away from one another. Lenslet 222 may evenpartially overlap one another on microlens array 220. Lenslet 222includes two characteristic dimensions: its width a and height (ordepth) b. Lenslet 222 may be characterized by its aspect ratio, whichfor the purposes of this application is given by b/a. Lenslet 222 mayinclude at least one curved surface. In some embodiments, lenslet 222may include only curved surfaces. Lenslet 222 as depicted in FIG. 2 may,at least for the purposes of this application be considered to be andreferred to as a concave microfeature.

The aspect ratio of lenset 222 may be important in designing a stackwith appropriate optical properties. Aspect ratio may in some cases be atradeoff between collimating effect and diffusion. For example, forlenslets, a relatively higher aspect ratio may provide comparativelybetter diffusion while providing less collimating effect. In someembodiments, an aspect ratio of about 0.5 is suitable. In someembodiments, an aspect ratio of less than 0.5, for example, about 0.3 issuitable. Individual lenslets within microlens array 220 may havevariations in aspect ratio beyond normal manufacturing variability. Insuch cases, the average aspect ratio may be less than 0.5, or any othersuitable value.

Microlens array 220 may be formed through any suitable process,including microreplication processes such as casting and curing. Theresin used to form microlens array 220 may be selected for itsprocessing capabilities, other physical properties such as durability,melt, and warp resistance, and optical properties such as index ofrefraction, transparency, and diffusive qualities. In some embodiments,the area between lenset 222 of microlens array 220 may be rounded,squared off, or otherwise modified to reduce potential scratching ofadjacent films.

Optical substrate 230 is disposed between collimating reflectivepolarizer 210 and microlens array 220. In some embodiments microlensarray 220 is microreplicated directly onto optical substrate 230.Optical substrate 230 and microlens array 220 may be the same material,and in some embodiments they may be part of the same monolithic piece ofmaterial. In other embodiments, optical substrate 230 may be adhered orlaminated to microlens array 220. Optical substrate may be any suitablematerial, including polymeric materials such as polyethyleneterephthalate (PET), polycarbonate (PC), or poly(methyl methacrylate)(PMMA). Optical substrate 230 may be any suitable dimensions, includingany suitable thickness. In some embodiments, the appropriate thicknessmay be chosen to provide suitable rigidity. In some embodiments opticalsubstrate 230 may have its thickness chosen to provide suitable opticalpath length between microlens array 220 and collimating reflectivepolarizer 210. Optical substrate 230 may be substantially transparentand optically inert, or it may have optical functionality such as beinga bulk diffuser, or including absorbing polarizing elements or dyes.

Adhesive 240 is optionally disposed between collimating reflectivepolarizer 210 and optical substrate 230. Adhesive 240 may be anysuitable adhesive, including an optically clear adhesive (OCA), apressure sensitive adhesive (PSA), or a heat or UV-curable adhesive. Insome embodiments, it may be desirable for adhesive 240 to be opticallyinert, i.e., transparent. In other embodiments adhesive 240 may includepigment, diffusing particles, or other optically active components whichalter or modify light travelling through optical film 200.

The overall configuration of optical film 200 depicted in FIG. 2 may beof any suitable size and may have any suitable dimensions. In someembodiments the layers of optical film 200 may be of the same size, andmay even be manufactured or stored in roll form, to be converted toparts of desired size. In general, optical film 200 has two outersurfaces: the first adjacent to collimating reflective polarizer 210,and the second adjacent to microlens array 220. Optical film 200 maycontain other films and optical components, such as a turning film,absorbing polarizer, or diffuser attached to the first outer surfaceadjacent to collimating reflective polarizer 210.

Concave microlens arrays are not generally used in conjunction withreflective polarizers in a recycling backlight. In particular, concavemicrolens arrays were thought to have a diffusing effect that is toostrong and undesirable in terms of backlight efficiency. Applicants havesurprisingly discovered, however, that in combination with a collimatingreflective polarizer, concave microlens arrays may provide desiredoverall collimation effects with adequate diffusion for backlight defecthiding. This allows for greater flexibility in backlight design andenables thinner constructions with comparable or superior performance asstandard backlight recycling cavities. Further, the thinner baseconstruction enables design flexibility in adding more optical features,components, or layers within a backlight while preserving acceptableoverall thickness.

FIG. 3 is a side elevation cross-section of another optical filmincluding a collimating reflective polarizer. Optical film 300 includescollimating reflective polarizer 310 and microlens array 320 includinglenslet 322. FIG. 3 depicts an alternative configuration of an opticalfilm, generally corresponding to optical film 200 in FIG. 2 , however,microlens array 320 is microreplicated or otherwise formed directly ontocollimating reflective polarizer 310.

Optical film 300 may be a naturally thinner construction because it doesnot include an optical substrate. The thinness may be desirable in someapplications, including handheld and mobile displays. Otherwise, theselection and design of microlens array 320 and collimating reflectivepolarizer 310 may take into account the same considerations and featuresas described above for the corresponding elements 210 and 220 in FIG. 2.

FIG. 4 is a side elevation cross-section of another optical filmincluding a collimating reflective polarizer. Optical film 400 includescollimating reflective polarizer 410 including beads 412A, prisms 412B,and lenses 412C, microlens array 420 including lenslet 422, opticalsubstrate 430, and adhesive 440. Optical film 400 in FIG. 4 generallycorresponds to optical film 200 in FIG. 2 , however FIG. 4 illustrates avariety of top-surface microstructures and microfeatures that may beprovided on certain embodiments.

Beads 412A may be disposed on a surface adjacent to collimatingreflective polarizer 410, and may perform several functions. In someembodiments, beads 412A can function as an anti-wetout, ananti-reflection, or anti-Newton ring layer. Appropriate bead size anddensity based on desired functionality will be apparent to those withskill in the art. In some embodiments, 8 μm beads (beads having a meandiameter of 8 μm) may be used. Smaller beads, even beads as small as 1μm or smaller may be appropriate in some embodiments. The beads may bedeposited in a binder or a solution or solvent, which in some cases maybe evaporated away, leaving the beads disposed on or adhered to thesurface of optical film 400. Beads 412A may also provide scratchresistance or protection for collimating reflective polarizer 410. Beads412A may either be transparent or opaque and can be of any suitablematerial.

Prisms 412B and lenses 412C are shown on optical film 400 to demonstratethe variety of surface structures possible in embodiments of the opticalfilm described herein. Prisms 412B, for example, may be useful tofurther collimate light. Any suitable pitch and arrangement of thesemicrofeatures is contemplated in conjunction with optical film 400.While FIG. 4 shows beads 412A, prisms 412B, and lenses 412C on the sameoptical film for illustrative convenience and is not intended tonecessarily suggest discrete sections of different microfeatures, insome embodiments more than one type of microfeature may be disposed onoptical film 400.

FIG. 5 is a side elevation cross-section of a portion of an edge-litbacklight including the optical film of FIG. 2 . Optical stack 500includes collimating reflective polarizer 510, microlens array 520including lenslet 522, optical substrate 530, adhesive 540, one or morelight sources 550, reflector 552, and lightguide 554.

Optical stack 500 shows an optical film corresponding essentially tooptical film 200 in FIG. 2 , including collimating reflective polarizer510, microlens array 520 including lenslet 522, optical substrate 530,and adhesive 540. In addition, optical stack 500 includes one or morelight sources 550, reflector 552, and lightguide 554.

The configuration of one or more light sources 550, reflector 552, andlightguide 554 are typical of an edge-lit system. In such a system,light is injected from one or more light sources 550 and enterslightguide 554. Injection of light from one or more light sources 550into lightguide 554 may utilize injection or collimation optics. Lightis transported within lightguide 554 through total internal reflectionat one or more interfaces. In FIG. 5 , the top surface of lightguide 554is depicted as being exposed to air. Light incident on thelightguide/air interface at more than the critical angle (calculatedfrom the well known Snell's law) is totally internally reflected.Reflector 552 also serves to transport the light and may be specularlyor semi-specularly reflective. In some embodiments, lightguide 554,reflector 552, or both may include diffusing or extracting features thathelp decouple transported light from lightguide 554 and cause it to beincident on the lightguide/air interface at less than the criticalangle. In this way, light from one or more light sources 550 is madeincident on the portion of optical stack 500 that includes the filmequivalent to optical film 200 described in conjunction with FIG. 2 .Light that may be reflected by collimating reflective polarizer 510 isreflected back down through lightguide 554 and is incident on reflector552 before being reflected back up toward the optical film. In this way,light is recycled to a more useable polarization state or incidenceangle, when it otherwise may be emitted (from the ultimate backlight orviewing device) at unusable or undesirable viewing angles or absorbed byother components within the backlight.

One or more light sources 550 may be any number of light sources,including CCFLs, LEDs, incandescent light bulbs, or any combinationthereof. One or more light sources 550 may in some embodiments emit overa limited range of wavelengths; in some embodiments, different lightsources of the one or more light sources may emit over differentwavelength ranges. In some embodiments, one or more light sources 550may emit substantially white light. In embodiments where one or morelight sources 550 includes LEDs, one or more light sources 550 may emita substantially Lambertian distribution of light.

Lightguide 554 may be constructed from any suitable material and may beof any suitable shape or size. For example, a lightguide may be formedfrom acrylic and may be planar, tapered, or curved.

Reflector 552 may be any suitable reflector. In some embodimentsreflector 552 may be a multilayer reflective film, such as EnhancedSpecular Reflector (ESR), available from 3M Company, St. Paul, Minn.

Optical stack 500 is oriented such that microlens array 520 is proximateto lightguide 554. The relative illustrated sizes of, for example, oneor more light sources 550 and lenslet 522 or lightguide 554 andcollimating reflective polarizer 510 are for ease of illustration andshould not be understood to suggest actual relative dimensions of thosecomponents.

FIG. 6 is a side elevation cross-section of a portion of a direct-litbacklight including the optical film of FIG. 2 . Optical stack 600includes collimating reflective polarizer 610, microlens array 620including lenslet 622, optical substrate 630, adhesive 640, one or morelight sources 650, and reflector 652.

Optical stack 600 shows an optical film corresponding essentially tooptical film 200 in FIG. 2 , including collimating reflective polarizer610, microlens array 620 including lenslet 622, optical substrate 630,and adhesive 640. In addition, optical stack 600 includes one or morelight sources 650 and reflector 652.

The configuration of one or more light sources 650 and reflector 652 aretypical of a direct-lit backlight. In such a system, lightguides aregenerally not used—instead, light from one or more light sources isdirectly incident, in FIG. 6 , for example, on the optical filmcorresponding to optical film 200 in FIG. 2 . In such cases, brightspots are possible if the light sources are not sufficiently diffused.Light is incident on the optical film. Light reflected by collimatingreflective polarizer 610 is reflected back to reflector 652, and thendirected back toward the optical film. As in the configuration of FIG. 6, light is recycled within this backlight cavity. Microlens array 620 isconfigured such that it is proximate one or more light sources 650.

EXAMPLES Comparative Example C-1

A film stack was prepared by placing a reflective polarizer(DBEF-D2-400) on top of a prism film (320 micron BEF) which was placedon top of a beaded PET diffuser film having a haze of 7% (all availablefrom 3M Company, St. Paul, Minn.). The film was tested as a backlightfilm with the diffuser layer closest to the backlight. Gain was measuredusing a SPECTRASCAN PR-650, available from Photo Reasearch, Inc.,Chatsworth, Calif. Haze was measured using a HAZE-GARD PLUS 4725hazemeter (available from BYK-Gardiner, Silver Springs, Md.) On-axisbrightness was measured using an ELDIM EZ CONTRAST 160R (available fromELDIM Corp., France). Results are given in Table 1. The film stack wasplaced above an LED backlight with the diffuser layer closest to thebacklight and the degree that the film was able to hide the LEDs wasvisually assessed. It was found that the film stack provided good hotspot hiding.

Comparative Example C-2

A collimating multilayer optical film (CMOF) was prepared as describedin Example 3 of PCT Patent Application Number US 2012/060485,incorporated by reference herein. In particular, using the feedblockmethod described in U.S. Patent App. Pub. No. 2011/0272849 entitled“Feedblock for Manufacturing Multilayer Polymeric Films”, filed May 6,2011, two packets of 275 layers each of alternating low and high indexpolymer layers were coextruded as a cast web and then stretched in atenter on a continuous film making line. The high index material was a90/10 coPEN (90% naphthalate units and 10% teraphthalate units). The lowindex material differed between packet 1 and packet 2 of themicrolayers. The low index material for packet 1 was a blend of PETg(EASTAR GN071 copolyester from Eastman Chemical, Kingsport, Tenn.) andan amorphous 55/45 coPEN (55% naphthlate units and 45% terephthalateunits). The blend ratio was adjusted so as to obtain an index of 1.589.The low index material for packet 2 was TRITAN FX150 copolyester fromEastman Chemical (Kingsport, Tenn.) and had a measured index of 1.554.All indices were measured at 633 nm.

The layer thickness values of the CMOF film were measured using AtomicForce Microscopy (AFM), and the layer thickness profiles 13 a and 13 bare shown in FIG. 7 for packets 1 and 2 respectively.

The ratio of pass axis light transmitted at 60 degrees to thetransmission value at 0 degrees is a good indication of the collimatingpotential of the film when used with a recycling backlight. Thetransmission spectra for p-polarized incident light and s-polarized weredetermined for incidence angles of 0 and 60 degrees. The averagetransmission values for each measurement (% T) were estimated byaveraging the transmission values of each spectrum between 420 nm and680 nm. No photopic weighting was used, although this could be appliedif desired. The average % T values for the CMOF film were as follows:for p-pol: 75% at 0 deg, dropping to 46% at 60 deg, and for s-pol: 75%at 0 deg dropping to 36% at 60 deg. The ratios of the values ofTpass60/Tpassnormal for p-pol and s-pol light were 0.62 and 0.48respectively.

Gain, haze and brightness were determined as in Comparative Example C-1and are reported in Table 1. The degree that the film was able to hidethe LEDs was assessed as in Comparative Example C-1 and was found to bevery poor.

Comparative Example C-3

A CMOF film was prepared as in Comparative Example C-2 and a beadeddiffuser layer (a bead coated 188 micron thick PET film having a 7%haze, available from SKC Haas Display Films (Seoul, Korea) under thetrade name CH003U0) was attached to the CMOF using OCA 8171 CL (anoptically clear adhesive available from 3M Company, St. Paul, Minn.)with the bead side facing away from the CMOF. The film was tested as abacklight film with the beaded layer closest to the backlight. Gain,haze and brightness were determined as in Comparative Example C-1 andare reported in Table 1. The degree that the film was able to hide theLEDs was assessed as in Comparative Example C-1 and was found to bepoor.

Example 1

A CMOF film was prepared as in Comparative Example C-1. A PET filmhaving concave microlenses on one surface was obtained from MNTchCorporation (South Korea). The microlenses had a pitch of about 30microns and a height of about 10 microns. The microlens film waslaminated to the CMOF film using OCA 8171 CL (available from 3M Company,St. Paul, Minn.) with the microlens side facing away from the CMOF. Thefilm was tested as a backlight film with the microlens layer closest tothe backlight. Gain, haze and brightness were determined as inComparative Example C-1. The degree that the film was able to hide theLEDs was assessed as in Comparative Example C-1 and was found to providegood hot spot hiding.

Example 2

A film was made as in Example 1 except that the microlenses had a pitchof about 50 microns and a height of about 25 microns. The film wastested as a backlight film with the microlens layer closest to thebacklight. Gain, haze and brightness were determined as in ComparativeExample C-1. The degree that the film was able to hide the LEDs wasassessed as in Comparative Example C-1 and was found to provide good hotspot hiding.

TABLE 1 On-axis On-axis Brightness, Haze Brightness percent of Comp.Example Gain (%) (nits) Ex. C-1 C-1 1.92 100 309 100 C-2 1.84 1 275 89C-3 1.80 71 255 83 1 1.89 100 295 95.5 2 1.79 100 279 90.3

The following are exemplary embodiments according to the presentdisclosure:

Item 1. An optical film, comprising:

-   -   a collimating reflective polarizer; and    -   an array of concave microlenses disposed on a major surface of        the collimating reflective polarizer.

Item 2. An optical film, comprising:

-   -   an optical substrate having a first and second major surface;    -   a collimating reflective polarizer disposed on the first major        surface of the optical substrate; and    -   an array of concave microlenses disposed on the second major        surface of the optical substrate.

Item 3. The optical film of item 2, further comprising an adhesivebetween the optical substrate and the collimating reflective polarizer.

Item 4. The optical film of item 1 or 2, wherein the collimatingreflective polarizer has a transmission along a pass axis at normalincidence of Tpassnormal for p-pol light and a transmission along a passaxis at 60 degrees incidence of Tpass60 for p-pol light, and a ratio ofTpass60 to Tpassnormal is less than 0.75.

Item 5. The optical film of item 4, wherein the ratio of Tpass60 toTpassnormal is less than 0.60.

Item 6. The optical film of item 5, wherein the ratio of Tpass60 toTpassnormal is less than 0.50.

Item 7. The optical film of item 1 or 2, wherein the array of concavemicrolenses is randomly arranged.

Item 8. The optical film of item 1 or 2, wherein the array of concavemicrolenses is regularly arranged.

Item 9. The optical film of item 1 or 2, wherein each microlens in thearray of concave microlenses has an aspect ratio of about 0.5.

Item 10. The optical film of item 9, wherein each microlens in the arrayof concave microlenses has an aspect ratio of about 0.3.

Item 11. The optical film of item 1 or 2, wherein each microlens in thearray of concave microlenses has an aspect ratio of less than about 0.5.

Item 12. The optical film of item 1, further comprising microfeaturesdisposed on a surface of the collimating reflective polarizer notdisposed on the array of concave microlenses.

Item 13. The optical film of item 2, further comprising microfeaturesdisposed on a surface of the collimating reflective polarizer notdisposed on the optical substrate.

Item 14. The optical film of item 12 or 13, wherein the microfeaturesinclude beads.

Item 15. The optical film of item 14, wherein a mean diameter of thebeads is about 8 μm.

Item 16. The optical film of item 12 or 13, wherein the microfeaturesinclude convex lenses.

Item 17. The optical film of item 12 or 13, wherein the microfeaturesinclude prisms.

Item 18. The optical film of item 1 or 2, wherein the optical film has athickness of less than 420 μm.

Item 19. An edge-lit backlight assembly, comprising:

-   -   the optical film of item 1 or 2; and    -   a lightguide;    -   wherein the lightguide is disposed proximate the array of        concave microlenses.

Item 20. A direct-lit backlight assembly, comprising:

-   -   the optical film of item 1 or 2; and    -   one or more light sources;    -   wherein the one or more light sources are disposed proximate the        array of concave microlenses.

Descriptions for elements in figures should be understood to applyequally to corresponding elements in other figures, unless indicatedotherwise. The present invention should not be considered limited to theparticular embodiments described above, as such embodiments aredescribed in detail in order to facilitate explanation of variousaspects of the invention. Rather, the present invention should beunderstood to cover all aspects of the invention, including variousmodifications, equivalent processes, and alternative devices fallingwithin the scope of the invention as defined by the appended claims andtheir equivalents.

What is claimed is:
 1. An optical stack comprising an array ofmicrolenses disposed on a reflective polarizer, such that when theoptical stack is incorporated into a light recycling backlightconfigured to provide light to a display panel, the microlenses and thereflective polarizer, in combination, collimate the light, wherein thereflective polarizer has a transmission along a pass axis at normalincidence of Tpassnormal for p-pol light and a transmission along thepass axis at 60 degrees incidence of Tpass60 for p-pol light, andwherein a ratio of Tpass60 to Tpassnormal is less than 0.75.
 2. Abacklight comprising a light source and the optical stack of claim 1disposed on a reflector, the reflector and the optical stack defining alight recycling cavity therebetween, the backlight configured to providethe light to the display panel, wherein the microlenses and thereflective polarizer, in combination, collimate the light.
 3. Thebacklight of claim 2 being an edge-lit backlight.
 4. The backlight ofclaim 2 being a direct-lit backlight.
 5. The optical stack of claim 1,wherein the array of the microlenses is a regular array.